Method and apparatus for determining a fingerprint of a performance parameter

ABSTRACT

A lithographic process is one that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. During the lithographic process, the focus should be controlled. There is disclosed a method for determining a fingerprint of a performance parameter associated with a substrate, such as a focus value to be used during the lithographic process. A reference fingerprint of the performance parameter is determined for a reference substrate. A reference substrate parameter of the reference substrate is determined. A substrate parameter for a substrate, such as a substrate with product structures, is determined. Subsequently, the fingerprint of the performance parameter is determined based on the reference fingerprint, the reference substrate parameter and the substrate parameter. The fingerprint may then be used to control the lithographic process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 16178809.6 which wasfiled on Jul. 11, 2016 and EP application 16195549.7 which was filed onOct. 21, 2016 which are incorporated herein in its entirety byreference.

FIELD

The present invention relates to a method and apparatus for determininga fingerprint of a performance parameter of a lithographic substrate.More particularly, the invention relates to a method and apparatus fordetermining a fingerprint of a focus parameter.

BACKGROUND

A lithographic process is one that applies a desired pattern onto asubstrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically done by imaging thepatterning device onto a layer of radiation-sensitive material (resist)provided on the substrate by way of an optical system (e.g. a projectionlens). Stepping and/or scanning movements can be involved, to repeat thepattern at successive target portions across the substrate. It is alsopossible to transfer the pattern from the patterning device to thesubstrate by imprinting the pattern onto the substrate.

An important property of interest is critical dimension (CD). It isimportant that structures are formed with accurate critical dimensioncontrol over the whole substrate (e.g. wafer). A key parameter in orderto control the critical dimension during the lithographic process is theposition of the substrate relative to the focal plane of thelithographic apparatus (which may also be known as the “focus setting”).In particular, control of the focus setting must be carefully maintainedduring exposure of the substrate. This may be achieved by controllingthe focus characteristics of the projection lens, and or by controllingthe position of the substrate such that it is kept close to the focalplane of the projection lens during exposure of the substrate.

Typically, focus settings are determined by performing measurements onone or more focus targets. The focus targets are positioned on apatterning device (e.g. reticle) and are patterned onto the substrateusing a lithographic step. Typically the patterning device alsocomprises structures associated with the pattern of the product (e.g. anIC), said structures referred to as “product structures”. Afterpatterning the product structures and focus targets are present on thesubstrate. The focus target are measured (for example in a metrology orinspection apparatus) and the focus setting is determined. Thedetermined focus setting is representative for a certain deviationbetween a reference and an actual focus setting during exposure of thesubstrate. Knowledge of the focus setting may be used to correct thelithographic apparatus focus in order to enhance performance of thelithographic process. This correction may be achieved by adjusting anoptical element within the projection lens of the lithographic apparatusor by adjusting the position and/or orientation of the substrate withrespect to the focal plane of the projection lens of the lithographicapparatus.

The determined focus setting is representative for a certain deviationbetween a desired and an actual focus setting during exposure of thesubstrate. Knowledge of the focus setting may be used to correct thelithographic apparatus focus in order to enhance performance of thelithographic process. This correction may be achieved by adjusting anoptical element within the projection lens of the lithographic apparatusor by adjusting the position and/or orientation of the substrate withrespect to the focal plane of the projection lens of the lithographicapparatus.

However, the focus targets take up space on the substrate. This directlyreduces the number of product structures that can be placed on asubstrate, which is undesirable. Additionally, the positioning andfeatures of the focus targets may cause interference with nearby productstructures, thereby potentially degrading the quality of these productstructures.

Further, in order to determine the focus setting it is necessary tocarry out test and calibration procedures in addition to themeasurements themselves. Furthermore, such measurements have to becarried out using a metrology or inspection apparatus. The substratesunder measurement are therefore delayed during the measurement process,which proportionally increases production time and thereby throughput ofthe lithographic apparatus.

The known method measures the total focus setting for a substrate, whichincludes all focus error sources. Therefore, it may be difficult toidentify the root cause of any defects or focus error sources whilstusing the above method and apparatus. The known method does notdistinguish between different sources of focus errors (e.g. errorscaused by the lithographic apparatus or errors caused by thelithographic process). Accordingly, identifying and correcting focuserrors and their source may take a significant amount of time.

Typically the used focus targets are placed on a product reticle (areticle comprising product structures) and comprise diffractivestructures having a pitch smaller than the pitch of the productstructures. After these focus targets are patterned (exposed in resist),the focus setting can be determined from diffraction based measurements.Basically the focus setting is reconstructed from the observeddiffraction pattern. This method of measuring a focus setting iscommonly referred to as “diffraction based focus” (DBF) measurement. Itsfocus targets are referred to as diffraction based focus targets (eg“DBF targets).

The fact that the focus targets and product structures are patternedduring the same lithographic process is essential. The focus targets areexposed at exactly the same conditions as the product structures (samedose settings, illumination mode, lens settings, stage characteristicsetc.). The measured focus settings are hence representative for focusbehavior of the lithographic apparatus during production, e.g. thedetermined focus setting is relevant for both focus target and productstructures.

The described diffraction based method to measure a focus setting wasfound to be less successful when the thickness of the resist was chosento be very thin. This is for example the case when adopting an ExtremeUltra-Violet (EUV) lithographic process for which the resist must bevery thin to prevent a too strong absorption gradient throughout theresist stack. In case of a thin resist stack, for example thinner than50-100 nm a diffraction based method will suffer as the radiation usedto perform the diffraction based metrology increasingly becomesreflected from structures underlying the resist pattern. In addition therequired pitch of the DBF targets scales with the product structurepitch. Sub-resolution pitches for a EUV process, as required for the DBFtargets, will become increasingly challenging in view ofmanufacturability of the DBF targets on the reticle.

The invention proposes a solution to measure a focus settingrepresentative for the lithographic process during production whenadopting a thin resist and/or high resolution lithographic process (forexample EUV or a low kl-DUV process).

SUMMARY

It is proposed to limit performing measurements on focus targets to areference substrate. In addition to the focus measurement also ameasurement associated with a property of the reference substrate isperformed, for example a height map of the reference substrate. Usingthe reference focus measurement and the measured height map allowsdetermination of a focus fingerprint for any product substrate for whicha height map has been determined. A direct focus measurement is then nolonger needed to determine focus settings associated with productsubstrates avoiding the use of space consuming focus targets. Inaddition to a focus setting also other parameters (referred to asperformance parameters) associated with the product substrate may bedetermined in a similar fashion.

In a first aspect, the invention provides a method for determining afingerprint of a performance parameter associated with a substrate, themethod comprising:

determining a reference fingerprint of the performance parameterassociated with a reference substrate;

determining at least one reference substrate parameter associated withthe reference substrate;

determining at least one substrate parameter associated with thesubstrate; and

determining the fingerprint of the performance parameter based on thereference substrate parameter, the substrate parameter and the referencefingerprint.

In a second aspect, the invention provides a method for manufacturingdevices, wherein device features are formed on a series of substrates bya lithographic process, wherein properties of the processed substratesare measured by one or more measuring processes, and wherein themeasured properties are used to determine a fingerprint of a performanceparameter according to a method as provided above.

The invention further provides a lithographic apparatus comprising meansfor carrying out a method for determining a fingerprint of a performanceparameter as provided above.

The invention further provides a computer program product containing oneor more sequences of machine-readable instructions for implementing amethod as provided above.

It is further proposed to precede a volume manufacturing phase of alithographic process by a focus setting determination phase in which aproduction reticle, including focus targets is used to pattern one ormore substrates. Typically the substrates pertain to one or moresend-ahead lots comprising send-ahead substrates. The substrates areexposed at identical conditions (settings lithographic apparatus) as theproduct substrates (substrates patterned during the volume manufacturingphase). The focus targets are based on features having a sufficientlylarge pitch to guarantee good manufacturability and accurate readout onmetrology tooling. To enhance the response of the focus target to afocus setting an amount of astigmatism is introduced within theprojection lens of the lithographic apparatus while patterning thesubstrates. The patterned substrates are subsequently measured on ametrology tool and the focus setting is determined based on themeasurements. The determined focus setting is used to optimize thelithographic apparatus focus setting during the volume manufacturingphase of the lithographic process. In this fashion it will not benecessary to use focus targets which are not compatible with athin-resist and/or high resolution lithographic process.

In a further aspect, the invention provides a method for patterning aplurality of substrates utilizing a lithographic apparatus, the methodcomprising: determining a focus setting based on a measurement on astructure on a substrate, wherein the substrate has been exposed by thelithographic apparatus at an aberration setting associated with anenhanced sensitivity of the measurement on the structure to variationsof the focus setting; and patterning the plurality of substratesutilizing the lithographic apparatus at a corrected focus setting basedon the determined focus setting.

The invention further provides a lithographic apparatus comprising meansfor carrying out the method for patterning a plurality of substrates.

The invention further provides a computer program product containing oneor more sequences of machine-readable instructions for implementing themethod for patterning a plurality of substrates.

Further aspects, features and advantages of the invention, as well asthe structure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus together with other apparatusesforming a production facility for semiconductor devices;

FIG. 2 illustrates the steps to expose a substrate in the dual stageapparatus of FIG. 1;

FIG. 3 depicts a method in accordance with a first embodiment of theinvention;

FIG. 4 illustrates a system for carrying out the method of FIG. 3;

FIG. 5 is a schematic diagram of a method for determining a common gridlayout for a plurality of measurement points; and

FIG. 6 shows an exemplary comparison between a known method and themethod of FIG. 3.

FIG. 7 depicts a functional overview of an embodiment describing a focussetting optimization method.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 at 200 shows a lithographic apparatus LA as part of an industrialfacility implementing a high-volume, lithographic manufacturing process.In the present example, the manufacturing process is adapted for themanufacture of semiconductor products (integrated circuits) onsubstrates such as semiconductor wafers. The skilled person willappreciate that a wide variety of products can be manufactured byprocessing different types of substrates in variants of this process.The production of semiconductor products is used purely as an examplewhich has great commercial significance today.

Within the lithographic apparatus (or “litho tool” 200 for short), ameasurement station MEA is shown at 202 and an exposure station EXP isshown at 204. A control unit LACU is shown at 206. In this example, eachsubstrate visits the measurement station and the exposure station tohave a pattern applied. In an optical lithographic apparatus, forexample, a projection system is used to transfer a product pattern froma patterning device MA onto the substrate using conditioned radiationand a projection system. This is done by forming an image of the patternin a layer of radiation-sensitive resist material.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. The patterning MA device maybe a mask or reticle, which imparts a pattern to a radiation beamtransmitted or reflected by the patterning device. Well-known modes ofoperation include a stepping mode and a scanning mode. As is well known,the projection system may cooperate with support and positioning systemsfor the substrate and the patterning device in a variety of ways toapply a desired pattern to many target portions across a substrate.Programmable patterning devices may be used instead of reticles having afixed pattern. The radiation for example may include electromagneticradiation in the deep ultraviolet (DUV) or extreme ultraviolet (EUV)wavebands. The present disclosure is also applicable to other types oflithographic process, for example imprint lithography and direct writinglithography, for example by electron beam.

The lithographic apparatus control unit LACU which controls all themovements and measurements of various actuators and sensors to receivesubstrates W and reticles MA and to implement the patterning operations.LACU also includes signal processing and data processing capacity toimplement desired calculations relevant to the operation of theapparatus. In practice, control unit LACU will be realized as a systemof many sub-units, each handling the real-time data acquisition,processing and control of a subsystem or component within the apparatus.

Before the pattern is applied to a substrate at the exposure stationEXP, the substrate is processed in at the measurement station MEA sothat various preparatory steps may be carried out. The preparatory stepsmay include mapping the surface height of the substrate using a levelsensor and measuring the position of alignment marks on the substrateusing an alignment sensor. The alignment marks are arranged nominally ina regular grid pattern. However, due to inaccuracies in creating themarks and also due to deformations of the substrate that occurthroughout its processing, the marks deviate from the ideal grid.Consequently, in addition to measuring position and orientation of thesubstrate, the alignment sensor in practice must measure in detail thepositions of many marks across the substrate area, if the apparatus isto print product features at the correct locations with very highaccuracy. The apparatus may be of a so-called dual stage type which hastwo substrate tables, each with a positioning system controlled by thecontrol unit LACU. While one substrate on one substrate table is beingexposed at the exposure station EXP, another substrate can be loadedonto the other substrate table at the measurement station MEA so thatvarious preparatory steps may be carried out. The measurement ofalignment marks is therefore very time-consuming and the provision oftwo substrate tables enables a substantial increase in the throughput ofthe apparatus. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations. Lithographic apparatus LA may for example is of aso-called dual stage type which has two substrate tables WTa and WTb andtwo stations—an exposure station and a measurement station—between whichthe substrate tables can be exchanged.

Within the production facility, apparatus 200 forms part of a “lithocell” or “litho cluster” that contains also a coating apparatus 208 forapplying photosensitive resist and other coatings to substrates W forpatterning by the apparatus 200. At an output side of apparatus 200, abaking apparatus 210 and developing apparatus 212 are provided fordeveloping the exposed pattern into a physical resist pattern. Betweenall of these apparatuses, substrate handling systems take care ofsupporting the substrates and transferring them from one piece ofapparatus to the next. These apparatuses, which are often collectivelyreferred to as the track, are under the control of a track control unitwhich is itself controlled by a supervisory control system SCS, whichalso controls the lithographic apparatus via lithographic apparatuscontrol unit LACU. Thus, the different apparatus can be operated tomaximize throughput and processing efficiency. Supervisory controlsystem SCS receives recipe information R which provides in great detaila definition of the steps to be performed to create each patternedsubstrate.

Once the pattern has been applied and developed in the litho cell,patterned substrates 220 are transferred to other processing apparatusessuch as are illustrated at 222, 224, 226. A wide range of processingsteps is implemented by various apparatuses in a typical manufacturingfacility. For the sake of example, apparatus 222 in this embodiment isan etching station, and apparatus 224 performs a post-etch annealingstep. Further physical and/or chemical processing steps are applied infurther apparatuses, 226, etc. Numerous types of operation can berequired to make a real device, such as deposition of material,modification of surface material characteristics (oxidation, doping, ionimplantation etc.), chemical-mechanical polishing (CMP), and so forth.The apparatus 226 may, in practice, represent a series of differentprocessing steps performed in one or more apparatuses.

As is well known, the manufacture of semiconductor devices involves manyrepetitions of such processing, to build up device structures withappropriate materials and patterns, layer-by-layer on the substrate.Accordingly, substrates 230 arriving at the litho cluster may be newlyprepared substrates, or they may be substrates that have been processedpreviously in this cluster or in another apparatus entirely. Similarly,depending on the required processing, substrates 232 on leavingapparatus 226 may be returned for a subsequent patterning operation inthe same litho cluster, they may be destined for patterning operationsin a different cluster, or they may be finished products to be sent fordicing and packaging.

Each layer of the product structure requires a different set of processsteps, and the apparatuses 226 used at each layer may be completelydifferent in type. Further, even where the processing steps to beapplied by the apparatus 226 are nominally the same, in a largefacility, there may be several supposedly identical machines working inparallel to perform the step 226 on different substrates. Smalldifferences in set-up or faults between these machines can mean thatthey influence different substrates in different ways. Even steps thatare relatively common to each layer, such as etching (apparatus 222) maybe implemented by several etching apparatuses that are nominallyidentical but working in parallel to maximize throughput. In practice,moreover, different layers require different etch processes, for examplechemical etches, plasma etches, according to the details of the materialto be etched, and special requirements such as, for example, anisotropicetching.

The previous and/or subsequent processes may be performed in otherlithography apparatuses, as just mentioned, and may even be performed indifferent types of lithography apparatus. For example, some layers inthe device manufacturing process which are very demanding in parameterssuch as resolution and overlay may be performed in a more advancedlithography tool than other layers that are less demanding. Thereforesome layers may be exposed in an immersion type lithography tool, whileothers are exposed in a ‘dry’ tool. Some layers may be exposed in a toolworking at DUV wavelengths, while others are exposed using EUVwavelength radiation.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly a manufacturing facility in which litho cell LC islocated also includes metrology system MET which receives some or all ofthe substrates W that have been processed in the litho cell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem SCS. If errors are detected, adjustments may be made to exposuresof subsequent substrates, especially if the metrology can be done soonand fast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped and reworkedto improve yield, or discarded, thereby avoiding performing furtherprocessing on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

Also shown in FIG. 1 is a metrology apparatus 240 which is provided formaking measurements of parameters of the products at desired stages inthe manufacturing process. A common example of a metrology apparatus ina modern lithographic production facility is a scatterometer, forexample an angle-resolved scatterometer or a spectroscopicscatterometer, and it may be applied to measure properties of thedeveloped substrates at 220 prior to etching in the apparatus 222. Usingmetrology apparatus 240, it may be determined, for example, thatimportant performance parameters such as focus, overlay or criticaldimension (CD) do not meet specified accuracy requirements in thedeveloped resist. Prior to the etching step, the opportunity exists tostrip the developed resist and reprocess the substrates 220 through thelitho cluster. As is also well known, the metrology results 242 from theapparatus 240 can be used to maintain accurate performance of thepatterning operations in the litho cluster, by supervisory controlsystem SCS and/or control unit LACU 206 making small adjustments overtime, thereby minimizing the risk of products being madeout-of-specification, and requiring re-work. The metrology apparatus 240and/or other metrology apparatuses (not shown) can be applied to measureproperties of the processed substrates 232, 234, and incoming substrates230.

FIG. 2 illustrates the steps to expose target portions (e.g. dies) on asubstrate W in the dual stage apparatus of FIG. 1.

On the left hand side within a dotted box are steps performed at ameasurement station MEA, while the right hand side shows steps performedat the exposure station EXP. From time to time, one of the substratetables WTa, WTb will be at the exposure station, while the other is atthe measurement station, as described above. For the purposes of thisdescription, it is assumed that a substrate W has already been loadedinto the exposure station. At step 200, a new substrate W′ is loaded tothe apparatus by a mechanism not shown. These two substrates areprocessed in parallel in order to increase the throughput of thelithographic apparatus.

Referring initially to the newly-loaded substrate W′, this may be apreviously unprocessed substrate, prepared with a new photo resist forfirst time exposure in the apparatus. In general, however, thelithography process described will be merely one step in a series ofexposure and processing steps, so that substrate W′ has been throughthis apparatus and/or other lithography apparatuses, several timesalready, and may have subsequent processes to undergo as well.Particularly for the problem of improving overlay performance, the taskis to ensure that new patterns are applied in exactly the correctposition on a substrate that has already been subjected to one or morecycles of patterning and processing. These processing stepsprogressively introduce distortions in the substrate that must bemeasured and corrected for, to achieve satisfactory overlay performance.

The previous and/or subsequent patterning step may be performed in otherlithography apparatuses, as just mentioned, and may even be performed indifferent types of lithography apparatus. For example, some layers inthe device manufacturing process which are very demanding in parameterssuch as resolution and overlay may be performed in a more advancedlithography tool than other layers that are less demanding. Thereforesome layers may be exposed in an immersion type lithography tool, whileothers are exposed in a ‘dry’ tool. Some layers may be exposed in a toolworking at DUV wavelengths, while others are exposed using EUVwavelength radiation.

At 202, alignment measurements using the substrate marks P1 etc. andimage sensors (not shown) are used to measure and record alignment ofthe substrate relative to substrate table WTa/WTb. In addition, severalalignment marks across the substrate W′ will be measured using alignmentsensor AS. These measurements are used in one embodiment to establish a“wafer grid”, which maps very accurately the distribution of marksacross the substrate, including any distortion relative to a nominalrectangular grid.

At step 204, a map of substrate height (Z) against X-Y position ismeasured also using the level sensor LS. Conventionally, the height mapis used only to achieve accurate focusing of the exposed pattern. Aswill be explained further below, the present apparatus uses height mapdata also to supplement the alignment measurements.

When substrate W′ was loaded, recipe data 206 were received, definingthe exposures to be performed, and also properties of the wafer and thepatterns previously made and to be made upon it. To these recipe dataare added the measurements of wafer position, wafer grid and height mapthat were made at 202, 204, so that a complete set of recipe andmeasurement data 208 can be passed to the exposure station EXP. Themeasurements of alignment data for example comprise X and Y positions ofalignment targets formed in a fixed or nominally fixed relationship tothe product patterns that are the product of the lithographic process.These alignment data, taken just before exposure, are combined andinterpolated to provide parameters of an alignment model. Theseparameters and the alignment model will be used during the exposureoperation to correct positions of patterns applied in the currentlithographic step. A conventional alignment model might comprise four,five or six parameters, together defining translation, rotation andscaling of the ‘ideal’ grid, in different dimensions. As describedfurther in US 2013230797A1, advanced models are known that use moreparameters.

At 210, wafers W′ and W are swapped, so that the measured substrate W′becomes the substrate W entering the exposure station EXP. In theexample apparatus of FIG. 1, this swapping is performed by exchangingthe supports WTa and WTb within the apparatus, so that the substrates W,W′ remain accurately clamped and positioned on those supports, topreserve relative alignment between the substrate tables and substratesthemselves. Accordingly, once the tables have been swapped, determiningthe relative position between projection system PS and substrate tableWTb (formerly WTa) is all that is necessary to make use of themeasurement information 202, 204 for the substrate W (formerly W′) incontrol of the exposure steps. At step 212, reticle alignment isperformed using the mask alignment marks M1, M2. In steps 214, 216, 218,scanning motions and radiation pulses are applied at successive targetlocations across the substrate W, in order to complete the exposure of anumber of patterns.

By using the alignment data and height map obtained at the measuringstation in the performance of the exposure steps, these patterns areaccurately aligned with respect to the desired locations, and, inparticular, with respect to features previously laid down on the samesubstrate. The exposed substrate, now labeled W″ is unloaded from theapparatus at step 220, to undergo etching or other processes, inaccordance with the exposed pattern.

As described above, performing focus measurements in the known mannerrequires specific target structures to be provided on the substrate. Thepresence of such target structures reduces the surface area on thesubstrate available for product structures. This reduces the number ofproduct structures that may be provided on the substrate, which directlyreduces the product yield and increases product costs. Additionally,determining the root cause of focus errors may be difficult since theknown method results in a total focus measurement for the substrate. Forexample, it is non-trivial to determine whether a focus error is due toa process-related effect or whether it is due to a lithographicapparatus-related effect.

Accordingly, it is desirable to omit the use of focus target structureson product substrates. However, this removes the ability to carry outthe above-described focus measurements, and in turn prevents thedetermination of the focus setting to be used by the lithographicapparatus in order to correct for any focus errors in the system. Inother terms, removing the focus target structures may significantlyreduce the accuracy of the lithographic apparatus.

Further, it is desirable to provide a method of determining a focussetting for a position on a substrate exposed in the lithographicapparatus, wherein it is possible to separate individual sources offocus errors. In other terms, it is desirable to determine theindividual factors or properties that may change the required focussetting, thereby potentially causing focus errors, for any givenposition on the substrate.

The inventors have realized that it is possible to determine a focussetting for the lithographic apparatus without requiring measurements tobe carried out on the above-described focus target structures on productsubstrates. Instead of using such target structures on the productsubstrates, which reduces the product yield, the focus setting may bederived from focus measurements performed on one or more referencesubstrates and one or more sets of focus-related measurement data.

The inventors have additionally realized that by using a plurality ofmeasurement data, it becomes possible to determine and separate the rootcauses of focus errors. This, in turn may lead to an improvement in thedesign processes for subsequent substrates.

An exemplary method for determining a fingerprint of a performanceparameter associated with a substrate will now be briefly outlined withreference to FIG. 3. A more detailed discussion will follow below withreference to both FIGS. 3 and 4.

In a first step 301, a reference fingerprint 402 of the performanceparameter associated with a reference substrate is determined. Thefingerprint of the performance parameter is determined by patterning anumber of measurement target structures from a patterning device ontothe reference substrate, and subsequently measuring one or moreproperties of the target structures. In one example, the targetstructures are designed to allow measurement of a focus setting. Thefocus setting may be measured in any suitable fashion, e.g. by using thelithographic apparatus or by using a dedicated measurement apparatus. Itwill be appreciated that, although described in the following withrespect to focus setting, other performance parameters may equally wellbe determined using the exemplary method. The measurement targetstructures may be designed to allow measurement of performanceparameters such as (without limitation): alignment, overlay, dose orcritical dimension (CD).

In a second step 302, a reference substrate parameter 404 associatedwith the reference substrate is determined. When the performanceparameter is a focus setting, the reference substrate parameter may, forexample, be a height map of the reference substrate (which may forexample be measured by a level sensor of the lithographic apparatus).Other reference substrate parameters may be relevant for performanceparameters associated with, e.g. overlay or CD performance, includingbut not limited to: alignment fingerprints (by measurement of alignmentmark positions across the reference substrate) or stack characteristicsassociated with the reference substrate. The stack characteristics mayfor example be related to resist thickness measurements or reflectivitymeasurements performed across the reference substrate.

In a third step 303, a substrate parameter 420 associated with thesubstrate (which is typically provided with one or more productstructures) is determined. In an example, the substrate parameter isdetermined in a substantially identical fashion to the referencesubstrate parameter, but is determined by measuring a substrate, such asa product substrate containing product features, rather than a referencesubstrate.

In a fourth step 304, a fingerprint 424 of the performance parameter isdetermined based on the reference fingerprint 402, the referencesubstrate parameter 404 and the substrate parameter 420. Knowledge ofthe fingerprint of the reference performance parameter and the referencesubstrate parameter, determined for the reference substrate, allows thefingerprint of the performance parameter for the substrate to bedetermined, as long as the substrate parameter has also been determined.This obviates the need for direct determination of the fingerprint ofthe performance parameter on the substrate. In the example where theperformance parameter is a focus setting, the above-described methodenables the focus setting to be determined without requiring focusmeasurements to be performed directly on the substrate, in turnobviating the need for focus target structures on the surface of thesubstrate.

It will be appreciated that, while described in respect of focussettings, the above method could, in principle, be used to determine anumber of different performance parameters of a substrate. Accordingly,the determination of a focus setting should be seen as exemplary only,and not be interpreted as limiting.

The above method will now be discussed in detail with reference to FIGS.3 and 4. As described, in the first step 301, the reference fingerprint402 of the performance parameter associated with a reference substrateis determined. In an example, the performance parameter is afocus-related parameter. In a specific example, the referencefingerprint is a focus setting that reflects a relative position of thereference substrate with respect to a focal plane of the lithographicapparatus. As described above, it should be noted that this is forexemplary purposes only. Examples wherein reference fingerprints aredetermined for other performance parameters (e.g. alignment, criticaldimension or overlay error) may be easily envisaged by the skilledperson.

In some examples, the reference fingerprint is determined based on oneor more specific contributions. Generally, contributions can be dividedinto two types: intra-field contributions and inter-field contributions.As described above, during exposure, the patterning device is imagedonto the surface of the substrate. Each of the exposed patterns on thesubstrate surface is referred to as a field. Intra-field contributionsrefer to contributions that repeat in each field, i.e. contributionsthat repeat for each individual exposure. Inter-field contributionsrefer to contributions that are distributed across a part of, or theentirety of, the substrate, i.e. they do not repeat for each field.

In an example, an inter-field contribution 406 to the referencefingerprint is derived based on a first set of reference measurementdata. The first set of measurement data may be obtained in any suitablefashion. As described, the inter-field contribution describessubstrate-wide contributions to the reference fingerprint. In otherterms, the inter-field contribution describes effects or contributionsthat vary across the entire surface of the substrate, including (but notlimited to): deformation of the substrate and/or the substrate stage onwhich the substrate is placed; or imperfections in the surface of thebase substrate. In one example, the first set of measurement datacomprises height data describing height variations due to imperfectionsin the surface of the reference substrate.

Additionally or alternatively, the reference fingerprint may bedetermined based on other contributions. In one such example, anintra-field contribution 408 to the reference fingerprint is derivedbased on a second set of reference measurement data. As described, theintra-field contribution describes contributions to the referencefingerprint that are repeated in each field, including (but not limitedto): deformation (e.g. bending) of the patterning device (reticle); ordeformations in the projection lens.

In some examples, the reference fingerprint may be determined based on aplurality of sets of reference measurement data. In an example, theplurality sets may be obtained by performing measurements on a pluralityof reference substrates over a specific period of time. This enables atime-dependent or time-evolving contribution 410 to the referencefingerprint to be derived and taken into account. It will be appreciatedthat the plurality of sets of reference measurement data may be used toderive one or both of inter-field and intra-field contributions. Forexample, in one example, the plurality of sets of reference measurementdata are used to derive an inter-field contribution. In another example,the plurality of sets of measurement data are used to derive both aninter-field and an intra-field contribution.

By performing reference measurements over a period of time, itadditionally becomes possible to determine focus stability for thereference substrates. Such stability information can then be used toemulate and/or predict focus stability for subsequent batches ofsubstrates.

In some examples, the reference measurement data comprises informationassociated with a characteristic of the lithographic apparatus. Asreference substrates typically do not comprise any product structures,the reference measurement data will predominantly reflect the propertiesof the various components of the lithographic apparatus. In one example,the information is associated with an optical characteristic of anoptical system of the lithographic apparatus. In a specific example, theinformation is representative of aberrations in one or more opticalelements (e.g. a lens) of an optical system, such as an exposure system,of the lithographic apparatus. In another example, the information isassociated with a characteristic of a positioning system of thelithographic apparatus. In one such example, the information isrepresentative of positioning errors for one or more movable elements orstages (e.g. a substrate stage) of the lithographic apparatus.

It will be appreciated that the reference measurement data may beobtained in any suitable fashion, using any suitable measurement systemor methodology. For example, when the information of the referencemeasurement data is representative of the optical properties of theoptical system of the lithographic apparatus, the information may beobtained either by measurement or calculation. In specific examples, thereference substrate has provided on its surface one or more focustargets that are used to perform focus measurements. In other examples,other types of targets or target structures may be used to performmeasurements. The measurement results are subsequently used to determineaberration in one or more of the lenses of the optical system of thelithographic apparatus. In other examples, the reference substratecomprises targets or features for position or alignment measurements(e.g. target structures for measuring the alignment of the patterningdevice), the result of which may in some examples be used to obtaininformation representative of positional deviations or errors in one ormore of the movable elements of the lithographic apparatus.

In the second step 302, the reference substrate parameter 404 associatedwith the reference substrate is determined. The reference substrateparameter may be determined in any suitable fashion. In some examples,the reference substrate parameter comprises a height map (or otherheight-related information) for the reference substrate. In a specificexample the height map comprises height measurement data obtained by alevel sensor. The height measurement data may be obtained at anyconvenient time. For example, the height measurement data may beobtained prior to the lithographic process, or it may be obtained aspart of the lithographic process. Alternatively, the height measurementdata may comprise both data obtained prior to the lithographic processand data obtained during the lithographic process.

In general the reference substrate parameter is chosen to be a heightmap when the performance parameter is (related to) a focus setting. Whenthe performance parameter is (related to) an overlay performance it ismore useful to incorporate alignment measurement data associated withthe reference substrate (as both overlay and alignment data relate topositions of features defined within the plane of the substrate). It ishence beneficial to use alignment measurement data as a referencesubstrate parameter. When the performance parameter is a criticaldimension one may use stack characteristics data associated with thereference substrate as a reference substrate parameter.

In some examples, the step of determining the reference substrateparameter further comprises obtaining additional reference data 405. Theadditional reference data may be obtained by performing additionalmeasurements, and may for example be performed in order to increase theaccuracy of the height map. For example, the additional reference datamay comprise correctional information representative of errors in theheight map for the reference substrate, e.g. substrate stage positioningerrors. In another example, the additional reference data for thereference substrate parameter comprises correctional data informationrepresentative of errors in the height map for the substrate (ratherthan the reference substrate). Increasing the accuracy of the height mapdirectly improves control of the yield of the lithographic apparatus.For example, increasing the accuracy of the height map allows animproved ability to correct for positional errors (including heighterrors), which decreases the variance of the height profile of thesubstrate. If the variance of the height profile is not controlled, therisk is increased that some of the product structures are not exposedcorrectly and may therefore not be of sufficient quality, which in turnnegatively impacts yield and increases the price of each productstructure.

The additional reference data may be obtained in any suitable fashion.In some examples, the additional reference data is obtained by an airgauge measurement performed at a suitable time during the lithographicprocess. Alternatively, in other examples, the additional reference datais obtained by logging positioning errors of one or more movablecomponents of the lithographic apparatus (e.g. substrate or reticlestage positioning errors), the latter being particularly relevant whenthe reference substrate parameter is related to alignment measurementdata.

The first and second steps, in unison, enables an apparatus fingerprint412 (which may also be referred to as a “scanner fingerprint”) that isrepresentative of the properties of the lithographic apparatus to bederived. The apparatus fingerprint may be derived in any suitablefashion. In an example, the apparatus fingerprint is derived bysubtracting the reference substrate parameter 404 and the additionalreference data 405 from the reference fingerprint 402:

apparatus fingerprint=reference fingerprint−(substrateparameter+additional reference data)

It will be appreciated that the contributors to the apparatusfingerprint described above are exemplary only, and should not beinterpreted in any limiting fashion. Other examples, wherein additionalor alternative contributors to the apparatus fingerprint are used, wouldbe easily envisaged by the skilled person.

In the third step 303, the substrate parameter 420 associated with thesubstrate (which is typically provided with one or more productstructures) is determined. The substrate parameter is, in some examples,determined in a similar fashion to the reference substrate parameter butfor the substrate rather than the reference substrate. Accordingly, itwill be appreciated that the various examples and method steps describedwith reference to the reference substrate parameter may also be appliedto the substrate parameter. Typically, the methodology used to obtainthe substrate parameter is substantially identical to that used toobtain the reference substrate parameter.

In some examples, the substrate parameter comprises a height map for thesubstrate. The height map may comprise height data for part of or forthe entirety of the surface of the substrate. In an example, the heightmap comprises height data obtained after at least a first patterningstep. In another example, the height map comprises height data obtainedafter at least a first processing step.

In yet other examples, the height map comprises a plurality of sets ofheight data, each set having been obtained after one of a plurality ofpatterning and/or further processing steps. It will be realized that, inprinciple, any suitable number of sets of height data may be obtained atany suitable time during the lithographic process. Further, it is, inprinciple, possible to utilize height data from previous substrates orbatches of substrates.

In some examples, the step of determining the substrate parameterfurther comprises obtaining additional substrate data 425, similarly tothe reference substrate parameter 404 described above. The substrateparameter and the additional substrate data may collectively be referredto as a processing fingerprint 422. The processing fingerprint may bedetermined in any suitable fashion. In one example, the processingfingerprint 422 is equal to the sum of the substrate parameter 420 andany additional substrate data 425:

processing fingerprint=substrate parameter+additional substrate data

In some examples, the additional substrate data additionally oralternatively comprises information representative of one or moreprocess-related effects. For example, product structures are becomingincreasingly complex with an increasing number of layers and using anincreasing number of different materials. Each material may have uniqueoptical properties. Some materials may be opaque at some wavelengths andtransparent at others. It is therefore necessary to compensate for anyerrors in measurements caused by such material-related effects. In someexamples, it is assumed that the process-related effects are constant(i.e. that the process dependency is stable over time). In suchexamples, the contribution relating to the process-related effects isconstant. However, in other examples, the process-related effects may bedetermined to vary over time or in dependence on process parameters.

In the fourth step 304, the fingerprint 424 of the performance parameteris determined based on the reference fingerprint 402, the referencesubstrate parameter 404 and the substrate parameter 420.

The fingerprint of the performance parameter may be determined in anysuitable fashion. In some examples, the fingerprint of the performanceparameter is obtained by a simple summation, i.e.:

performance parameter=processing fingerprint+apparatus fingerprint

It will be appreciated that this is merely exemplary and non-limiting.Many specific determination methodologies may be envisaged by theskilled person. For example, the fingerprint of the performanceparameter may be obtained by using the above-described summation, towhich one or more further parameters are added (e.g. the additionalparameter described in the following).

It will be appreciated that the characteristics and parameters used todetermine the fingerprint of the performance parameter described in theabove are exemplary only. It is, in principle, equally possible todetermine the fingerprint of the performance parameter by usingalternative or additional parameters and characteristics.

In some examples, in addition to the above-described characteristics andparameters, one or more additional characteristics used to determine thefingerprint of the performance parameter. This may, for example, includecharacteristics that exhibit temporal variations, or it may includecharacteristics that are otherwise dependent on a substrate map orindividual field maps. The additional characteristics may be used toderive an additional fingerprint 426 for the substrate. The additionalfingerprint may also be referred to as a “temporal fingerprint”.

In one example, the additional fingerprint 426 comprises movementinformation 428 of one or more movable components of the lithographicapparatus. For example, the movement information may be associated withmovements of a level sensor during a measurement phase. In a specificexample, movement information associated with the differences between amovement pattern of the level sensor when performing measurements on areference substrate and a movement pattern of the level sensor whenperforming measurements on a product substrate. In another example, themovement information is associated with movement of the patterningdevice during an exposure step. For example, the movement informationmay specifically be associated with movement errors during the exposurestep, e.g. differences between programmed movements and actualmovements.

It will be noted that the above-described movements of the level sensorand/or patterning device are relative to the substrate undermeasurement. As such, it will be realized that the level sensor orpatterning device could equally well be stationary while the substrateis moved. Alternatively, both of the level sensor or patterning deviceand the substrate may be moved during the measurements.

In other examples, the additional fingerprint comprises one or moretemporal characteristics 430 of the optical system of the lithographicapparatus. In one such example, the temporal characteristic isassociated with temperature-dependent changes in optical properties ofone or more optical components of the optical system of the lithographicapparatus. It is well know that the optical properties of opticalcomponents, and in particular optical lenses, may be dependent on thetemperature of the component (this effect may sometimes be referred toas “lens heating”).

In yet other examples, the additional fingerprint comprises a one ormore physical characteristics 432 of a component of the lithographicapparatus. In one such example, the additional fingerprint comprisescharacteristics of the patterning device, e.g. variations in ordeformations of the patterning device. As explained above, the patternon the patterning device is transferred to the substrate during thelithographic process. Any defects or deformations of the patterningdevice therefore affect the quality of the pattern transferred to thesubstrate. For example, deformations of the patterning device willinfluence the focus setting of the lithographic apparatus. Hence, ifsuch variations can be determined, it becomes possible to modify thefocus setting accordingly, thereby increasing the accuracy of thelithographic apparatus.

It will be realized that, in addition to the above-describedcharacteristics and parameters, it is possible to use statistical dataand/or data obtained during previous measurements 434. Such data may,for example, have been obtained from previous substrates or batches ofsubstrates. In this manner, it becomes possible to identify, and correctfor, variations between batches of substrates.

It will be appreciated that all of the sets of data described above(such as, but not limited to the reference fingerprint, referencesubstrate parameter, substrate parameter as well as the additionalfingerprint) consist of a plurality of discrete data points arranged ina convenient manner. In some examples, data points are arranged inregular grid layouts. In other examples, data points are laid out tocover specific portions of a substrate surface, such as a criticalproduct structure or component.

In the above, it has, purely for exemplary purposes, been assumed thatthe data points of each of the sets of data has been arranged in anidentical grid layout. In reality, this may not always be the case, andit will therefore be necessary to modify the sets of data to enable thefingerprint of the performance parameter to be determined. An example ofsuch a process, which will also be referred to as “re-gridding”, isillustrated in FIG. 5. It will be appreciated that the grid layoutsshown in Figure are purely for exemplary purposes.

FIG. 5 illustrates a situation wherein three sets of data, such asmeasurement or calculation results, are used. A first set of data 502 isarranged in a first grid layout 504. Similarly, a second set of data 506is arranged in a second grid layout 508, and a third set of data 510 isarranged in a third grid layout 512. As can be seen, each of the threegrid layouts differ from the other two grid layouts.

FIG. 5 further shows a fourth grid layout 514. For comparison with thefirst, second and third grid layouts, the data points 516 of the fourthgrid layout are shown as with dotted lines overlaid on these gridlayouts. As can be seen, in order to conform to the fourth grid layout,it is necessary to at least partly derive new data points for each ofthe first, second and third sets of data. This derivation may beperformed in any suitable fashion. In some examples, the derivation maybe performed by linear interpolation. It will be realized that, inprinciple, any suitable interpolation methodology may be utilized.

The properties of the fourth grid layout, e.g. the horizontal andvertical distance between data points on the surface of a substrate, maybe determined in a suitable manner. In the example illustrated in FIG.5, the fourth grid layout has a horizontal spacing that is equivalent tothe second grid layout and a vertical spacing that is equivalent to thefirst grid layout. It will however be appreciated that this is purelyfor exemplary purpose, and that the skilled person may envisage othergrid layouts. In one example, the data points of the first, second andthird grid layouts are re-gridded to match the grid layout the lowestdensity of data points.

Returning to FIG. 4, the re-gridding steps may be carried out at anysuitable time during the measurement process illustrated in FIG. 4.However, typically, the re-gridding step 436 is performed immediatelyprior to the determination of the fingerprint of the performanceparameter.

FIG. 6 illustrates the correlation between actual focus settingmeasurement results, as relating to dedicated focus targets (along theX-axis 602) and calculated focus settings that have been derived usingthe above-described method (along the Y-axis 604). The line 606 shows alarge degree of correlation between measured and calculated results. Ascan be seen from FIG. 6, the above-described method is useful todetermine focus settings across a substrate without performing dedicatedmeasurements on focus target structures.

The described embodiments so far intend to omit the use of focus targetson a product reticle; e.g. the reticle comprising product structures. Areference reticle is used comprising focus targets which, once printedon the reference substrate, are measured using a metrology tool based onanalysis of a diffraction pattern generated by interaction ofmeasurement beam and the printed focus target. This is generallyconsidered to be an optimal method to determine a focus setting or focusfingerprint associated with a lithographic apparatus or lithographicprocess.

As described above, performing focus measurements in the known mannerrequires focus targets to be compatible with metrology tools based onscatterometry and/or diffraction pattern measurements. For a thin resistand/or high resolution process (EUV based process) the architecture ofthe diffraction/scatterometer based focus targets becomes impractical. Asolution has been proposed in a patent application, application numberPCT/EP2016/062259 (not published yet at the moment of writing), whereina diffraction based focus target is replaced with a focus targetconsisting of two features, each having a different orientation. Anexample of such a focus target is a target composed of a horizontallyoriented line (or space) and a vertically oriented line (or space), asprovided on a reticle comprising product structures. The reticlecomprising the focus targets is exposed while introducing an astigmaticaberration ‘AST’ in the projection lens of the lithographic apparatus.The astigmatic aberration causes a different defocussing effect of thehorizontal feature with respect to the vertical feature. This ismodelled as a shift of a Bossung curve associated with the horizontalfeature with respect to a Bossung curve associated with the verticalfeature. A Bossung curve expresses the response of a feature parameter(typically a Critical Dimension, often abbreviated to CD) as a functionof a magnitude of a deviation of a focus setting ‘F’ with respect to areference. In general a Bossung curve can be approximated to a quadraticrelation of a parameter (CD) with respect to the focus setting ‘F’:CD=a*(F−b){circumflex over ( )}2+c. The parameter ‘a’ is related to thecurvature of the Bossung, the parameter ‘b’ is related to a focus shiftof a Bossung with respect to a reference focus level and the parameter‘c’ is a target CD of the feature. Introduction of an astigmaticaberration will change the position of the focal plane of the projectionlens causing a shift of the Bossung; the parameter ‘b’ will depend onthe level of introduced AST. When the astigmatic aberration has axes ofsymmetry aligned to the orientation of the features(horizontal-vertical) a focus shift ‘b’ for a horizontal feature H and afocus shift ‘−b’ (opposite sign) for a vertical feature V will beinduced. The CD of feature H (CD_H) depends then on F according to:CD_H=a*(F-b){circumflex over ( )}2+c and the CD of feature V (CD_V)according to CD_V=a*(F+b){circumflex over ( )}2+c. When subtracting bothCD's the following relation is obtained: CD_H−CD_V=d*F, the parameter‘d’ being a constant. By measuring the difference in CD between thehorizontal and the vertical feature it is possible to reconstruct thefocus setting ‘F’ which is often associated with a focus error of thelithographic apparatus. It needs to be mentioned that instead ofmeasuring a CD (using a scanning electron microscope SEM or ascatterometer) also another parameter demonstrating a Bossung curvebehaviour through focus may be determined. When using a scatterometermetrology tool this parameter may be associated with an intensitydistribution of a zero order light beam after interaction with the focustarget. When using a metrology tool based on diffractive measurements aparameter based on a comparison between the energy and/or phase contentof the various orders (−1, +1, 0, etc.) may be used.

The focus setting ‘F’ is defined with respect to a reference focussetting, generally corresponding to the focus setting for which theBossung curve demonstrates a maximum or minimum value of the CD or otherparameter of interest. It is emphasized that the focus setting in thisdocument is always associated with a mode of operation of thelithographic apparatus before introduction of the introduced astigmatismerror. The astigmatic aberration does alter a focus setting of alithographic apparatus, but when referring to the focus setting a focussetting related to effects other than the deliberately introducedastigmatic aberration are meant.

In addition to introduction of an astigmatic aberration also otheraberrations may be introduced in case their introduction enhances thefocus sensitivity of the structures used for measurement. For examplespherical aberrations may be selected in case a focus target is based ona phase shift mask principle. The aberration may be introduced by othermethods than manipulation of the projection lens. For example thereticle height (distance between the reticle and the focal plane of theprojection lens, as measured along the optical axis of the projectionlens) may be adjusted in order to introduce spherical aberrations.

The inventors have realized that it is possible to use the abovedescribed principle of determining a focus setting of the lithographicapparatus in a volume production environment. The concept is illustratedin FIG. 7. The arrows demonstrate the sequence of steps and productswhen adopting a method of volume production as proposed by theinventors. The steps and arrows which are dashed refer to optionalsteps.

A focus setting 1300 for the lithographic apparatus is determined basedon a measurement 1200. The measurement 1200 is performed on a focustarget on a substrate 1010 which has been exposed during a step 1100.The exposure 1100 is performed while an aberration 1105 (typically AST)is introduced. The focus target may be chosen from many structures whichare sensitive to the focus setting of the lithographic apparatus, inparticular when an aberration (astigmatic, spherical aberration) isintroduced.

In one embodiment the focus target structures are lines & spacesoriented along two or more directions, for example a pair of lines &spaces. One structure oriented along a horizontal and the otherstructure along a vertical direction.

In another embodiment the focus target is a single feature for which afirst metric associated with a first orientation and a second metricassociated with a second orientation are determined. An example of thisembodiment is the selection of a contact hole as a single feature forwhich a horizontal and a vertical dimension are determined. Analog tothe previously described case of a target composed out of horizontal andvertical lines a metric based on subtraction of the second parameterfrom the first parameter may be used to obtain a metric dependinglinearly on a deviation of the focus setting from the reference focussetting.

In another embodiment instead of dedicated focus target structures,product structures are chosen to determine the focus setting. This isuseful if among the product structures features exist which, incombination with the introduced astigmatism, may be used to derive afocus setting of the lithographic apparatus. For example a (CD)measurement on a horizontally oriented and a vertically oriented featureof a product structure may render this a viable solution. The advantageis that in this case no focus targets need to be provided to thereticle, leaving more design freedom and a larger usable area for theproduct structures.

In another embodiment a time evolving aspect of the focus setting needsto be determined. It is then required to extend the focus settingdetermination across more than one substrate. After exposure of thesubstrate ‘i’ a substrate ‘i+1’ will be selected for an exposure 1100and measurement 1200 to determine a focus setting 1300 representativefor the lithographic apparatus during exposure of the substrate ‘i+1’.Exposure of multiple substrates may be useful as during exposure(optical) components of the lithographic apparatus may heat up causing asignificantly different focus behavior of the lithographic apparatus.Components like the reticle, projection lens and the substrate itselfare prone to heating effects cause by prolonged usage of thelithographic apparatus (exposures). The light used to expose the reticlecauses heating of the reticle, projection lens and the substrate. Theresult is a drift of the focus setting of the lithographic apparatus. Byselecting multiple substrates (e.g. repeat step 1005 of selectingsubstrate ‘i+1’, wherein ‘i’ refers to the ID of the last exposedsubstrate) for exposure and subsequent focus setting measurements atypical focus setting evolution during exposure of a (production) lot ofsubstrates may be established.

In another embodiment a model is fitted to the focus setting evolutioncharacteristics. This model may for example be a focus fingerprint(spatial distribution of a focus setting across the substrate) whereineach focus setting value is modelled as a parameterized exponentialfunction in time.

In another embodiment the dose of the radiation used to pattern thesubstrates 1000 used to determine the focus setting is higher than thedose used during a patterning step of the product substrates 1400. Theeffect being a stronger heating effect of the components leading to areduction in substrates 1000 needed to establish a time evolution aspectof the focus setting 1300.

In another embodiment a selection of substrates 1000 out of a lot ofsubstrates is selected for subsequent exposure 1100, measurement 1200and determination of a focus setting 1300.

In another embodiment one or more substrates out of a plurality of lotsof substrates are selected for subsequent exposure 1100, measurement1200 and determination of a focus setting 1300.

In another embodiment the substrates 1000 used to determine the focussetting 1300 pertain to one or more send-ahead lots comprisingsend-ahead substrates. As during exposure of the product reticle anastigmatic aberration 1105 was introduced, in many cases the patternedsubstrates are only useful to determine a focus setting associated withthe lithographic apparatus. Focus critical product features are likelyto be affected by the deliberately introduced astigmatism and thepatterned substrates need to be reworked. This concept is generallyassociated with the ‘send-ahead’ principle; send-ahead substrates 1000from a send-ahead lot are exposed and measured in order to provideimproved machine settings for which subsequent (production) substrates1400 are to be exposed.

In another embodiment the substrates used to determine the focus settingare included within the set of patterned product substrates. When theproduct structures are not susceptible to astigmatic aberrations (forexample when they are unidirectional) the measured substrates may beincluded within the set of production substrates. This is illustrated inFIG. 7 by the dashed arrow between the measurement step 1200 and the boxrepresenting the end product; the patterned production substrates 1410.

After the focus setting 1300 has been determined the lithographicapparatus will be configured to start the patterning step 1500 ofproduct substrates 1400. Typically the product substrates will beexposed at a different aberration setting than during the exposure 1100of the substrates 1000. The aberration setting of the lithographicapparatus will be updated during a step 1510, although this is notnecessary in case of a limited susceptibility of the product structuresto astigmatic aberrations.

In another embodiment a correction of the focus setting is applied basedon the determined focus setting 1300. This correction may involveadjustment of the projection lens and/or positioning of the substrate1400 closer to or more parallel to the focal plane of the projectionlens. When the focus setting 1300 includes a model describing a temporalevolution characteristic the correction may be dynamical; e.g. the focussetting is adjusted according to an exposure history of the lithographicapparatus.

In another embodiment complimentary focus setting data (other than data1300) will be used to configure the exposure 1500 of the productsubstrates 1400. This additional focus setting data is referred to ascontext data 1600. The context data 1600 may comprise substrate geometrydata, levelling data, aberration measurement data, knowledge oftopographies of layers on the substrate (based on product structurelayouts) or focus setting data associated with substrates measured onother lithographic apparatus.

In another embodiment, apart from the aberration settings 1510, theconfiguration of the lithographic apparatus is not altered between theexposures 1100 and the exposures 1500. The illumination mode,illumination dose settings, reticle and substrate table parameters aresubstantially kept identical in order to keep the determined focussetting 1300 as representative as possible for the actual productionconditions during the product exposures 1500. Underlying the choice tokeep settings equal between exposures 1100 and 1500 is that the heatingcharacteristics of the reticle, projection lens and substrate need to besubstantially identical during focus setting determination and volumeproduction conditions.

In another embodiment the exposure 1100 and measurement 1200 of thesubstrates is done once, or on a very long time interval (weeks, months)as typically the focus setting behavior of the lithographic apparatusduring exposure of a lot of substrates will be stable during regularproduction. This generally applies for both short term evolutioncharacteristics (focus drifts during exposure of one lot) or long termevolution characteristics (focus drifts during exposure of a largenumber of lots; e.g. during a period longer than one day).

Further embodiments of the invention are listed in the followingnumbered clauses:

-   1. A method for determining a fingerprint of a performance parameter    associated with a substrate, the method comprising:    -   A step of determining a reference fingerprint of the performance        parameter associated with a reference substrate;    -   A step of determining at least one reference substrate parameter        associated with the reference substrate;    -   A step of determining at least one substrate parameter        associated with the substrate; and    -   A step of determining the fingerprint of the performance        parameter based on the first reference substrate parameter, the        substrate parameter and the reference fingerprint.-   2. A method according to clause 1, the method further comprising a    step of adjusting operation of a lithographic apparatus based on the    fingerprint of the performance parameter.-   3. A method according to clause 1 or 2, wherein the performance    parameter is a focus setting reflecting a relative position of the    substrate or reference substrate with respect to a focal plane of a    lithographic apparatus.-   4. A method according to clause 3, wherein the reference fingerprint    is determined by measurement of the focus setting across the    reference substrate.-   5 A method according to any preceding clause, wherein the reference    substrate parameter comprises a reference height map for the    reference substrate.-   6. A method according to clause 5, wherein the reference height map    comprises height measurement data obtained by a level sensor.-   7. A method according to clause 6, wherein the step of determining    the reference substrate parameter further comprises obtaining    additional reference data.-   8. A method according to clause 7, wherein the additional reference    data comprises correctional information representative of errors in    the reference height map.-   9. A method according to clause 7, wherein the additional reference    data comprises correction information representative of errors in a    substrate height map.-   10 A method according to any preceding clause, wherein the substrate    parameter comprises a substrate height map for the substrate.-   11. A method according to clause 10, wherein the substrate height    map comprises height data for the substrate obtained after at least    a first patterning step.-   12. A method according to clause 10 or 11, wherein the substrate    height map comprises height data for the substrate obtained after at    least a first processing step.-   13. A method according to any of clauses 10-12, wherein the    substrate parameter comprises a plurality of substrate height maps    representing a corresponding plurality of patterned layers on the    substrate.-   14. A method according to any preceding clause, wherein the step of    determining the fingerprint of the performance parameter comprises    performing a summation of each of the reference fingerprint,    reference substrate parameter, and the substrate parameter.-   15. A method according to any preceding clause, further comprising a    step of adjusting at least one of the reference fingerprint,    reference substrate parameter and the substrate parameter so as to    cause the data points of each of the reference fingerprint,    reference substrate parameter and the substrate parameter to overlap    in a reference grid on the surface of the substrate.-   16. A method according to any preceding clause, further comprising a    step of determining an additional fingerprint of at least one    parameter of the substrate or reference substrate.-   17. A method according to any preceding clause, wherein the step of    determining a reference fingerprint comprises deriving an    inter-field contribution to the reference fingerprint based on a    first set of reference measurement data.-   18. A method according to any preceding clause, wherein the step of    determining a reference fingerprint comprises deriving an    intra-field contribution to the reference fingerprint based on a    second set of reference measurement data.-   19. A method according to any preceding clause, wherein the step of    determining a reference fingerprint comprises deriving a reference    fingerprint based on a plurality of sets of reference measurement    data.-   20. A method according to any of clauses 17-19, wherein the    reference measurement data comprises information associated with a    characteristic of the lithographic apparatus.-   21. A method according to clause 20, wherein the information is    associated with an optical characteristic of an optical system of    the lithographic apparatus.-   22. A method according to clause 20 or 21, wherein the information    is associated with a characteristic of a positioning system of the    lithographic apparatus.-   23. A method for manufacturing devices, wherein device features are    formed on a series of substrates by a lithographic process, wherein    properties of the processed substrates are measured by one or more    measuring processes, and wherein the measured properties are used to    determine a fingerprint of a performance parameter according to the    method of any of clauses 1 to 22.-   24. A lithographic apparatus comprising means for carrying out the    method of any of clauses 1 to 23.-   25. A lithographic apparatus according to clause 24, comprising    -   an illumination optical system arranged to illuminate a pattern,        and    -   a projection optical system arranged to project an image of the        pattern onto a substrate,    -   wherein the lithographic apparatus is arranged to use the        determined fingerprint of the performance parameter in applying        the pattern to further substrates.-   26. A computer program product containing one or more sequences of    machine-readable instructions for implementing the method of any of    clauses 1 to 23.-   27. A method according to clause 1 or 2, wherein the performance    parameter is an overlay related parameter.-   28. A method according to clause 27, wherein the reference    fingerprint is determined by measurement of overlay features across    the reference substrate.-   29. A method according to clause 27 or 28, wherein the reference    substrate parameter comprises an alignment mark position map for the    reference substrate.-   30. A method according to clause 29, wherein the alignment mark    position map comprises alignment measurement data obtained by an    alignment sensor.-   31. A method according to clause 1 or 2, wherein the performance    parameter is a critical dimension related parameter.-   32. A method according to clause 31, wherein the reference    fingerprint is determined by measurement of critical dimensions    across the reference substrate.-   33. A method according to clause 31 or 32, wherein the reference    substrate parameter comprises a stack property map for the reference    substrate.-   34. A method according to clause 33, wherein the stack property map    comprises resist thickness data.-   35. A method according to clause 33 or 34, wherein the stack    property map comprises reflectivity data.-   36. A method for patterning a plurality of substrates utilizing a    lithographic apparatus, the method comprising:    -   determining a focus setting based on a measurement on a        structure on a substrate, wherein the substrate has been exposed        by the lithographic apparatus at an aberration setting        associated with an enhanced sensitivity of the measurement on        the structure to variations of the focus setting; and    -   patterning the plurality of substrates utilizing the        lithographic apparatus at a corrected focus setting based on the        determined focus setting.-   37. A method according to clause 36, wherein the aberration setting    is associated with an introduction of astigmatism.-   38. A method according to clause 36 or clause 37, wherein a metric    is defined being representative for a deviation of the focus setting    from a target focus setting.-   39. A method according to clause 38, wherein the structure is a    focus target.-   40. A method according to any of clauses 38 to 39, wherein the    structure comprises two features.-   41. A method according to clause 40, wherein the metric is based on    a first metric associated with a first feature and a second metric    associated with a second feature.-   42. A method according to any of clauses 36 to 41, wherein the    structure is a product structure.-   43. A method according to any of clauses 36 to 42, wherein the    plurality of substrates is patterned utilizing the lithographic    apparatus at a second aberration setting different from the    aberration setting associated with an enhanced sensitivity of the    measurement to variations of the focus setting.-   44. A method according to any of clauses 36 to 43, wherein the    measurement is based on analysis of a diffraction pattern of the    structure.-   45. A method according to any of clauses 36 to 44, wherein the    measurement is based on analysis of an image of the structure on the    substrate acquired by a metrology tool based on electron beam    imaging.-   46. A method according to any of clauses 36 to 45, wherein the    measurement is based on scatterometry.-   47. A method according to any of clauses 36 to 46, wherein the    plurality of substrates pertain to one or more lots of substrates.-   48. A method according to clause 47, wherein the lots of substrates    are associated with a volume manufacturing process.-   49. A method according to any of clauses 36 to 48, wherein the    measurement is performed on a structure on one or more substrates    pertaining to a send-ahead lot.-   50. A method according to any of clauses 36 to 49, wherein the    measurement is performed on a plurality of focus targets on one or    more substrates.-   51. A method according to clause 50, wherein the focus setting    includes a model establishing a temporal behavior of the focus    setting during the step of patterning the plurality of substrates.-   52. A method according to clause 51, wherein the corrected focus    setting is dynamically adjusted during said patterning of the    plurality of substrates.-   53. A method according to any of clauses 36 to 52, wherein the    corrected focus setting is based on the determined focus setting and    additional context data.-   54. A method according to clause 53, wherein the context data    comprises one or more of: leveling data, substrate geometry data,    alignment data, aberration data, reticle data.-   55. A method according to clause 54, wherein the measurement is    performed on a substrate pertaining to the plurality of substrates.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography, atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used in relation to the lithographicapparatus encompass all types of electromagnetic radiation, includingultraviolet (UV) radiation (e.g., having a wavelength of or about 365,355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation(e.g., having a wavelength in the range of 5-20 nm), as well as particlebeams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A method for determining a fingerprint of a performance parameterassociated with a substrate, the method comprising: determining areference fingerprint of the performance parameter associated with areference substrate; determining at least one reference substrateparameter associated with the reference substrate; determining at leastone substrate parameter associated with the substrate; and determiningthe fingerprint of the performance parameter based on the referencesubstrate parameter, the substrate parameter and the referencefingerprint.
 2. The method according to claim 1, further comprisingadjusting operation of a lithographic apparatus based on the fingerprintof the performance parameter.
 3. The method according to claim 1,wherein the performance parameter is a focus setting reflecting arelative position of the substrate or reference substrate with respectto a focal plane of a lithographic apparatus.
 4. The method according toclaim 3, wherein the reference fingerprint is determined by measurementof the focus setting across the reference substrate.
 5. The methodaccording to claim 1, wherein the reference substrate parametercomprises a reference height map for the reference substrate.
 6. Themethod according to claim 5, wherein the reference height map comprisesheight measurement data obtained by a level sensor.
 7. The methodaccording to claim 6, wherein determining the reference substrateparameter further comprises obtaining additional reference data.
 8. Themethod according to claim 1, wherein determining the fingerprint of theperformance parameter comprises performing a summation of each of thereference fingerprint, reference substrate parameter, and the substrateparameter.
 9. The method according to claim 1, further comprisingadjusting one or more selection from: the reference fingerprint, thereference substrate parameter and/or the substrate parameter, so as tocause the data points of each of the reference fingerprint, thereference substrate parameter and the substrate parameter to overlap ina reference grid on the surface of the substrate. 10.-15. (canceled) 16.The method according to claim 1, further comprising determining anadditional fingerprint of at least one parameter of the substrate orreference substrate.
 17. The method according to claim 1, whereindetermining a reference fingerprint comprises deriving an inter-fieldcontribution to the reference fingerprint based on a set of referencemeasurement data.
 18. The method according to claim 1, whereindetermining a reference fingerprint comprises deriving an intra-fieldcontribution to the reference fingerprint based on a set of referencemeasurement data.
 19. The method according to claim 1, wherein thereference fingerprint comprises information associated with acharacteristic of the lithographic apparatus.
 20. The method accordingto claim 19, wherein the information is associated with an opticalcharacteristic of an optical system of the lithographic apparatus. 21.The method according to claim 1, wherein the performance parameter is anoverlay related parameter.
 22. The method according to claim 1, whereinthe performance parameter is a critical dimension related parameter. 23.The method according to claim 22, wherein the reference fingerprint isdetermined by measurement of critical dimension values across thereference substrate and wherein the reference substrate parametercomprises a stack property map for the reference substrate.
 24. A methodfor manufacturing devices, wherein device features are formed on aseries of substrates by a lithographic process, wherein one or moreproperties of the processed substrates are measured by one or moremeasuring processes, and wherein the measured one or more properties areused to determine a fingerprint of a performance parameter according tothe method of claim
 1. 25. A lithographic apparatus comprising means forcarrying out the method of claim
 1. 26. A non-transitory computerprogram product containing one or more sequences of machine-readableinstructions, that when executed by a computer system, are configured tocause the computer system to at least: determine a reference fingerprintof a performance parameter associated with a reference substrate;determining at least one reference substrate parameter associated withthe reference substrate; determine at least one substrate parameterassociated with a substrate; and determine the fingerprint of theperformance parameter associated with the substrate based on thereference substrate parameter, the substrate parameter and the referencefingerprint.